For an HF molecule find the number of rotational levels located between the zeroth and first excited

Question:

For an HF molecule find the number of rotational levels located between the zeroth and first excited vibrational levels assuming rotational states to be independent of vibrational ones. The natural vibration frequency of this molecule is equal to 7.79.1014 rad/s, and the distance between the nuclei is 91.7 pm.